Directionally specific changes in arterial pressure induce differential patterns of fos expression in discrete areas of the rat brainstem: a double-labeling study for Fos and catecholamines

Cilostazol induces C-fos expression in the trigeminal nucleus caudalis and behavioural changes suggestive of headache with the migraine-like feature photophobia in female rats

Introduction

Research in development of new migraine therapeutics is hindered by the lack of suitable, predictive animal models. Cilostazol provokes headache in healthy humans and migraineurs by increasing intracellular cAMP levels. We aimed to investigate whether cilostazol could provoke headache-like behaviours and c-fos expression in rats. In order to evaluate the predictive validity of the model, we examined the response to the migraine specific drug sumatriptan.

Methods

The effect of cilostazol (125 mg/kg p.o.) in female Sprague Dawley rats was evaluated on a range of spontaneous behavioural parameters, light sensitivity and mechanical sensitivity thresholds. We also measured c-fos expression in the trigeminal nucleus caudalis.

Results

Cilostazol increased light sensitivity and grooming behaviour. These manifestations were not inhibited by sumatriptan. Cilostazol also induced c-fos expression in the trigeminal nucleus caudalis. Furthermore, trigeminal – but not hind paw hyperalgesia was observed.

Conclusion

The altered behaviours are suggestive of cilostazol induced headache with migraine-like features, but not specific. The presence of head specific hyperalgesia and the c-fos response in the trigeminal nucleus caudalis imply that the model involves trigeminal nociception. The model will be useful for studying mechanisms related to the cAMP pathway in headache, but its predictive properties appear to be more limited due to the lack of response to sumatriptan.

Angiotensin II AT1 receptors mediate neuronal sensitization and sustained blood pressure response induced by a single injection of amphetamine

A single exposure to amphetamine induces neurochemical sensitization in striatal areas. The neuropeptide angiotensin II, through AT₁ receptors (AT₁-R) activation, is involved in these responses. However, amphetamine-induced alterations can be extended to extra-striatal areas involved in blood pressure control and their physiological outcomes. Our aim for the present study was to analyze the possible role for AT₁-R in these events using a two-injection protocol and to further characterize the proposed AT₁-R antagonism protocol. Central effect of orally administered AT₁-R blocker (Candesartan, 3mg/kg p.o.×5days) in male Wistar rats was analyzed by spontaneous activity of neurons within locus coeruleus. In another group of animals pretreated with the AT₁-R blocker or vehicle, sensitization was achieved by a single administration of amphetamine (5mg/kg i.p. - day 6) followed by a 3-week period off drug. On day 27, after receiving an amphetamine challenge (0.5mg/kg i.p.), we evaluated: (1) the sensitized c-Fos expression in locus coeruleus (LC), nucleus of the solitary tract (NTS), caudal ventrolateral medulla (A1) and central amygdala (CeAmy); and (2) the blood pressure response. AT₁-R blockade decreased LC neurons' spontaneous firing rate. Moreover, sensitized c-Fos immunoreactivity in TH+neurons was found in LC and NTS; and both responses were blunted by the AT₁-R blocker pretreatment. Meanwhile, no differences were found neither in CeAmy nor A1. Sensitized blood pressure response was observed as sustained changes in mean arterial pressure and was effectively prevented by AT₁-R blockade. Our results extend AT₁-R role in amphetamine-induced sensitization over noradrenergic nuclei and their cardiovascular output.

Hypovolemic hemorrhage induces Fos expression in the rat hypothalamus: Evidence for involvement of the lateral hypothalamus in the decompensatory phase of hemorrhage

This study tested the hypothesis that the hypothalamus participates in the decompensatory phase of hemorrhage by measuring Fos immunoreactivity and by inhibiting neuronal activity in selected hypothalamic nuclei with lidocaine or cobalt chloride. Previously, we reported that inactivation of the arcuate nucleus inhibited, but did not fully prevent, the fall in arterial pressure evoked by hypotensive hemorrhage. Here, we report that hemorrhage (2.2 ml/100g body weight over 20 min) induced Fos expression in a high percentage of cells in the paraventricular, supraoptic and arcuate nuclei of the hypothalamus as shown previously. Lower densities of Fos immunoreactive cells were also found in the medial preoptic area (mPOA), anterior hypothalamus, lateral hypothalamus (LH), dorsomedial hypothalamus, ventromedial hypothalamus (VMH) and posterior hypothalamus. Bilateral injection of lidocaine (2%; 0.1 μl or 0.3 μl) or cobalt chloride (5mM; 0.3 μl) into the tuberal portion of the LH immediately before hemorrhage was initiated reduced the magnitude of hemorrhagic hypotension and bradycardia significantly. Lidocaine injection into the VMH also attenuated the fall in arterial pressure and heart rate evoked by hemorrhage although inactivation of the mPOA or rostral LH was ineffective. These findings indicate that hemorrhage activates neurons throughout much of the hypothalamus and that a relatively broad area of the hypothalamus, extending from the arcuate nucleus laterally through the caudal VMH and tuberal LH, plays an important role in the decompensatory phase of hemorrhage.

CGRP infusion in unanesthetized rats increases expression of c-Fos in the nucleus tractus solitarius and caudal ventrolateral medulla, but not in the trigeminal nucleus caudalis

BACKGROUND AND AIMS

Calcitonin gene-related peptide (CGRP) and glyceryl trinitrate (GTN) infusion in migraineurs provokes headache resembling spontaneous migraine, and CGRP receptor antagonists are effective in the treatment of acute migraine. We hypothesized that CGRP infusion would increase molecular markers of neuronal activation in migraine-relevant tissues of the rat.

METHODS

CGRP was infused intravenously (i.v.) in freely moving rats to circumvent factors like anesthesia, acute surgery and severe hypotension, the three confounding factors for c-Fos expression. The trigeminal nucleus caudalis (TNC) was isolated at different time points after CGRP infusion. The level of c-Fos mRNA and protein expression in TNC were analyzed by qPCR and immunohistochemistry. c-Fos-stained nuclei were also counted in the nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM), integrative sites in the brain stem for processing cardiovascular signals. We also investigated Zif268 protein expression (another immediate early gene) in TNC. The protein expression of p-ERK, p-CREB and c-Fos was analyzed in dura mater, trigeminal ganglion (TG) and TNC samples using Western blot.

RESULTS

CGRP infusion caused a significant dose-dependent fall in mean arterial blood pressure. No significant activation of c-Fos in the TNC at mRNA and protein levels was observed after CGRP infusion. A significant increase in c-Fos protein was observed in the NTS and CVLM in the brain stem. Zif268 expression in the TNC was also not changed after CGRP infusion. p-ERK was increased in the dura mater 30 minutes after CGRP infusion.

CONCLUSION

CGRP infusion increased the early expression of p-ERK in the dura mater but did not increase c-Fos and Zif268 expression in the TNC. The rats may, thus, differ from migraine patients, in whom infusion of CGRP caused headache and a delayed migraine attack. The rat CGRP infusion model with c-Fos or Zif268 as neuronal pain markers in TNC is unsuitable for antimigraine drug testing.

Efferent projections of C3 adrenergic neurons in the rat central nervous system

C3 neurons constitute one of three known adrenergic nuclei in the rat central nervous system (CNS). While the adrenergic C1 cell group has been extensively characterized both physiologically and anatomically, the C3 nucleus has remained relatively obscure. This study employed a lentiviral tracing technique that expresses green fluorescent protein behind a promoter selective to noradrenergic and adrenergic neurons. Microinjection of this virus into the C3 nucleus enabled the selective tracing of C3 efferents throughout the rat CNS, thus revealing the anatomical framework of C3 projections. C3 terminal fields were observed in over 40 different CNS nuclei, spanning all levels of the spinal cord, as well as various medullary, mesencephalic, hypothalamic, thalamic, and telencephalic nuclei. The highest densities of C3 axon varicosities were observed in Lamina X and the intermediolateral cell column of the thoracic spinal cord, as well as the dorsomedial medulla (both commissural and medial nuclei of the solitary tract, area postrema, and the dorsal motor nucleus of the vagus), ventrolateral periaqueductal gray, dorsal parabrachial nucleus, periventricular and rhomboid nuclei of the thalamus, and paraventricular and periventricular nuclei of the hypothalamus. In addition, moderate and sparse projections were observed in many catecholaminergic and serotonergic nuclei, as well as the area anterior and ventral to the third ventricle, Lamina X of the cervical, lumbar, and sacral spinal cord, and various hypothalamic and telencephalic nuclei. The anatomical map of C3 projections detailed in this survey hopes to lay the first steps toward developing a functional framework for this nucleus.

Lesions of the caudal ventrolateral medulla block the hypertension-induced inhibition of noxious-evoked c-fos expression in the rat spinal cord

The effect of lesioning the lateral portion of the caudal ventrolateral medullary reticular formation (VLMIat) on the noxious-evoked expression of the c-fos proto-oncogene in spinal neurons, was studied in short-term hypertensive rats. Occlusion of the renal artery for 96 h in unlesioned animals induced a 52% increase in blood pressure (BP) and a 66% decrease in the number of Fos-immunoreactive (Fos-IR) spinal cells following noxious cutaneous stimulation, as compared to values in normotensive controls. Lesioning the VLMIat in hypertensive rats by unilateral quinolinic acid (QA) injection (0.3 microl of a 180 nmol/microl solution) 24 h before noxious stimulation, prevented the Fos-IR cell decrease. In normotensive rats, lesioning the VLMIat produced no changes in c-fos expression. To investigate the role played by the VLMIat in cardiovascular control, BP and heart rate (HR) were measured during local injections of QA or glutamate (0.5 microl of a 100 nmol/microl solution) to normotensive animals. Injections of QA produced an immediate rise in BP and HR which reached maximal values (18 and 14% increase, respectively) 5 min after the administration onset, then returning gradually to baseline levels. Glutamate injections resulted in an immediate decrease of the same values, which reached 29 and 39%, respectively, 4 min after the beginning of injection, after which they decreased to baseline levels. These results suggest that VLMIat neurons inhibit nociceptive spinal neurons in response to rises in blood pressure, while exerting negative control of cardiovascular parameters. It is suggested that the VLMIat is involved in the genesis of hypoalgesia during hypertension.

Ascending caudal medullary catecholamine pathways drive sickness-induced deficits in exploratory behavior: brain substrates for fatigue?

Immune challenges can lead to marked behavioral changes, including fatigue, reduced social interest, anorexia, and somnolence, but the precise neuronal mechanisms that underlie sickness behavior remain elusive. Part of the neurocircuitry influencing behavior associated with illness likely includes viscerosensory nuclei located in the caudal brainstem, based on findings that inactivation of the dorsal vagal complex (DVC) can prevent social withdrawal. These brainstem nuclei contribute multiple neuronal projections that target different components of autonomic and stress-related neurocircuitry. In particular, catecholaminergic neurons in the ventrolateral medulla (VLM) and DVC target the hypothalamus and drive neuroendocrine responses to immune challenge, but their particular role in sickness behavior is not known. To test whether this catecholamine pathway also mediates sickness behavior, we compared effects of DVC inactivation with targeted lesion of the catecholamine pathway on exploratory behavior, which provides an index of motivation and fatigue, and associated patterns of brain activation assessed by immunohistochemical detection of c-Fos protein. LPS treatment dramatically reduced exploratory behavior, and produced a pattern of increased c-Fos expression in brain regions associated with stress and autonomic adjustments paraventricular hypothalamus (PVN), bed nucleus of the stria terminalis (BST), central amygdala (CEA), whereas activation was reduced in regions involved in exploratory behavior (hippocampus, dorsal striatum, ventral tuberomammillary nucleus, and ventral tegmental area). Both DVC inactivation and catecholamine lesion prevented reductions in exploratory behavior and completely blocked the inhibitory LPS effects on c-Fos expression in the behavior-associated regions. In contrast, LPS-induced activation in the CEA and BST was inhibited by DVC inactivation but not by catecholamine lesion. The findings support the idea that parallel pathways from immune-sensory caudal brainstem sources target distinct populations of forebrain neurons that likely mediate different aspects of sickness. The caudal medullary catecholaminergic projections to the hypothalamus may significantly contribute to brain mechanisms that induce behavioral "fatigue" in the context of physiological stressors.

Immune challenge and satiety-related activation of both distinct and overlapping neuronal populations in the brainstem indicate parallel pathways for viscerosensory signaling

Caudal brainstem viscerosensory nuclei convey information about the body's internal state to forebrain regions implicated in feeding behavior and responses to immune challenge, and may modulate ingestive behavior following immune activation. Illness-induced appetite loss might be attributed to accentuated "satiety" pathways, activation of a distinct "danger channel" separate from satiety pathways, or both. To evaluate neural substrates that could mediate the effects of illness on ingestive behavior, we analyzed the pattern and phenotypes of medullary neurons responsive to consumption of a preferred food, sweetened milk, and to intraperitoneal lipopolysaccharide challenge that reduced sweetened milk intake. Brainstem sections were stained for c-Fos, dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, and glucagon-like peptide-1 (GLP-1) immunoreactivity. Sweetened milk intake activated many neurons throughout the nucleus of the solitary tract (NTS), including A2 noradrenergic neurons in the caudal half of the NTS. LPS challenge activated a similar population of neurons in the NTS, in addition to rostral C2 adrenergic and mid-level A2 noradrenergic neurons in the NTS, many C1 and A1 neurons in the ventrolateral medulla, and in GLP-1 neurons in the dorsal medullary reticular nucleus. Increased numbers of activated GLP-1 neurons in the NTS were only associated with sweetened milk ingestion. Evidence for parallel processing was reflected in the parabrachial nucleus, where sweetened milk intake resulted in activation of the inner external lateral, ventrolateral and central medial portions, whereas LPS challenge induced c-Fos expression in the outer external lateral portions. Thus, signals generated in response to potentially dangerous physiological conditions seem to be propagated via specific populations of catecholaminergic neurons in the NTS and VLM, and likely include a pathway through the external lateral PBN. The data indicate that immune challenge engages multiple ascending neural pathways including both a distinct catecholaminergic "danger" pathway, and a possibly multimodal pathway derived from the NTS.

Dynamic transcriptomic response to acute hypertension in the nucleus tractus solitarius

Baroreceptor afferents project to the cardiovascular region of the nucleus tractus solitarius (cvNTS), and their cvNTS target neurons may play a role in governing the sensitivity and operating range of the arterial baroreceptor reflex (baroreflexes). Recent studies have shown differential gene and protein expression in the cvNTS in response to changed arterial pressure. However, the extent of these responses is unknown. Therefore, we collected differential global gene expression data in a time series following acute hypertension in awake, freely moving rats. To acquire statistically significant results and place them in functional context, we overcame several quality control requirements and developed novel analytical approaches. The physiologically new findings from the study are that acute hypertension causes very extensive, time-varying gene regulatory changes, many involving neuronal function-specific genes and systems of genes. We use standard genomic analysis methods to manage the large data sets and to develop results such as heat maps to examine patterns and clusters in the gene regulation. We used the Gene Ontology categories to provide functional context. To place our findings in the context of the relevant literature, we developed two graphical representations of the networks implicated, linking receptors and channels to signaling pathways. The results point to the multivariate complexity of the response and implicate a group of receptors as candidates for mediating nucleus tractus solitarius baroreflex function in hypertension by identifying concurrent upregulation of receptor genes. We were able to make transcription factor binding predictions and record dysregulation of heart rate correlated with the transcriptional response.

Visceral afferents directly activate catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 neurons are catecholamine (CA) neurons within the solitary tract nucleus (NTS) that influence many homeostatic functions, including cardiovascular reflexes, food intake, and stress. Because NTS is a major interface between sensory visceral afferents and the CNS, NTS CA neurons are ideally suited to coordinate complex responses by their projections to multiple brain regions. To test how NTS CA neurons process visceral afferent information carried by solitary tract (ST) afferents, we identified CA neurons using transgenic mice expressing TH-EGFP (enhanced green fluorescent protein under the control of the tyrosine hydroxylase promoter) and recorded synaptic responses to ST activation in horizontal slices. ST shocks evoked large-amplitude, short-latency, glutamatergic EPSCs (ST-EPSCs) in 90% of NTS CA neurons. Within neurons, ST-EPSCs had constant latency, rarely failed, and depressed substantially at high ST frequencies, indicating that NTS CA neurons receive direct monosynaptic connections from afferent terminals. NTS CA neurons received direct ST inputs from only one or two afferent fibers, with one-half also receiving smaller amplitude indirect inputs. Up to 90% of ST shocks evoked action potentials in NTS CA neurons. However, transmission of sensory afferent information through NTS CA neurons critically depended on the expression of an A-type potassium current (I(KA)), which when active attenuated ST-activated action potentials to a 37% success rate. The satiety peptide, cholecystokinin, presynaptically facilitated glutamate transmission in one-half of NTS CA neurons. Thus, NTS CA neurons are directly driven by visceral afferents with output being modulated by presynaptic peptide receptors and postsynaptic potassium channels.

Chronic intermittent hypoxia impairs baroreflex control of heart rate but enhances heart rate responses to vagal efferent stimulation in anesthetized mice

Chronic intermittent hypoxia (CIH) leads to increased sympathetic nerve activity and arterial hypertension. In this study, we tested the hypothesis that CIH impairs baroreflex (BR) control of heart rate (HR) in mice, and that decreased cardiac chronotropic responsiveness to vagal efferent activity contributes to such impairment. C57BL/6J mice were exposed to either room air (RA) or CIH (6-min alternations of 21% O(2) and 5.7% O(2), 12 h/day) for 90 days. After the treatment period, mice were anesthetized (Avertin) and arterial blood pressure (ABP) was measured from the femoral artery. Mean ABP (MABP) was significantly increased in mice exposed to CIH (98.7 +/- 2.5 vs. RA: 78.9 +/- 1.4 mmHg, P < 0.001). CIH increased HR significantly (584.7 +/- 8.9 beats/min; RA: 518.2 +/- 17.9 beats/min, P < 0.05). Sustained infusion of phenylephrine (PE) at different doses (0.1-0.4 microg/min) significantly increased MABP in both CIH and RA mice, but the ABP-mediated decreases in HR were significantly attenuated in mice exposed to CIH (P < 0.001). In contrast, decreases in HR in response to electrical stimulation of the left vagus nerve (30 microA, 2-ms pulses) were significantly enhanced in mice exposed to CIH compared with RA mice at low frequencies. We conclude that CIH elicits a sustained impairment of baroreflex control of HR in mice. The blunted BR-mediated bradycardia occurs despite enhanced cardiac chronotropic responsiveness to vagal efferent stimulation. This suggests that an afferent and/or a central defect is responsible for the baroreflex impairment following CIH.

Hemodynamic responses and c-Fos changes associated with hypotensive hemorrhage: standardizing a protocol for severe hemorrhage in conscious rats

The central mechanisms underlying the transition from compensation to decompensation during severe hemorrhage (HEM) are poorly understood. Furthermore, a lack of consistency in HEM protocols exists in the current literature. This study assessed the cardiovascular response and Fos-like immunoreactivity (FLI) in specific brain regions following severe HEM at three rates (2, 1, or 0.5 ml.kg(-1).min(-1)) in conscious rats. Heart rate (HR) and arterial pressure were recorded during the withdrawal of 30% of total blood volume (TBV). Data from animals hemorrhaged at the fast (F-HEM, n = 6), intermediate (I-HEM, n = 7), or slow (S-HEM, n = 7) rates were compared with saline (SAL, n = 5) and hypotensive (hydrazaline-induced, HYDRAZ, n = 5) controls. All HEM rates produced similar degrees of hypotension at the time of 30% TBV withdrawal. All HEM rates also produced bradycardia, but the change in HR was only significant in the F-HEM and I-HEM groups. Associated with I-HEM and F-HEM, but not HYDRAZ treatment were significant increases in FLI in the caudal ventrolateral periaqueductal gray (PAG), the central lateral nucleus of the rostral parabrachial nucleus, and locus coeruleus compared with SAL treatment. I-HEM also induced significant increases in FLI in the dorsomedial PAG, A7 region, and the cuneiform nucleus compared with SAL. S-HEM did not induce any significant change in FLI. Our results suggest that HEM at a rate of 1 ml.kg(-1).min(-1) may be most useful for investigating the potential role of the rostral brainstem regions in mediating hemorrhagic decompensation in conscious rats.

Catecholaminergic neurons in medullary nuclei are among the post-synaptic targets of descending projections from infralimbic area 25 of the rat medial prefrontal cortex

The infralimbic (IL) 'visceromotor' area of the rat medial prefrontal cortex projects to strategic subcortical nuclei involved in autonomic functions. Central among these targets are the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (rVLM). By combining tract-tracing using the anterograde tracer biotinylated dextran amine (BDA) with immunolabeling for tyrosine hydroxylase (TH; an enzyme marker of catecholaminergic neurons), a limited proportion of BDA-labeled IL axonal boutons in the NTS and rVLM was found to be closely associated with TH immunopositive (+) target structures. Such structural appositions were mainly located proximally over the labeled dendritic arbors of identified TH+ neurons. Quantitative ultrastructural examination revealed that in NTS, TH+ dendritic shafts comprised 7.0% of the overall post-synaptic target population innervated by BDA-labeled IL boutons, whereas TH+ dendritic spines represented 1.25% of targets. In rVLM, TH+ shafts represented 9.0% and TH+ spines 2.5% of IL targets. Labeled IL boutons established exclusively asymmetric Gray Type 1 (presumed excitatory) synaptic junctions. The results indicate that subpopulations of catecholaminergic neurons in the NTS and rVLM are among the spectrum of post-synaptic neurons monosynaptically innervated by descending 'excitatory' input from IL cortex. Such connectivity, albeit restricted, identifies the potential direct influence of IL cortex on the processing and distribution of cardiovascular, respiratory and related autonomic information by catecholaminergic neurons in the NTS and VLM of the rat.

Divergent projections of catecholaminergic neurons in the nucleus of the solitary tract to limbic forebrain and medullary autonomic brain regions

The nucleus of the solitary tract (NTS) is a critical structure involved in coordinating autonomic and visceral activities. Previous independent studies have demonstrated efferent projections from the NTS to the nucleus paragigantocellularis (PGi) and the central nucleus of the amygdala (CNA) in rat brain. To further characterize the neural circuitry originating from the NTS with postsynaptic targets in the amygdala and medullary autonomic targets, distinct green or red fluorescent latex microspheres were injected into the PGi and the CNA, respectively, of the same rat. Thirty-micron thick tissue sections through the lower brainstem and forebrain were collected. Every fourth section through the NTS region was processed for immunocytochemical detection of tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. Retrogradely labeled neurons from the PGi or CNA were distributed throughout the rostro-caudal segments of the NTS. However, the majority of neurons containing both retrograde tracers were distributed within the caudal third of the NTS. Cell counts revealed that approximately 27% of neurons projecting to the CNA in the NTS sent collateralized projections to the PGi while approximately 16% of neurons projecting to the PGi sent collateralized projections to the CNA. Interestingly, more than half of the PGi and CNA-projecting neurons in the NTS expressed TH immunoreactivity. These data indicate that catecholaminergic neurons in the NTS are poised to simultaneously coordinate activities in limbic and medullary autonomic brain regions.

Possible inhibitory role of prolactin-releasing peptide for ACTH release associated with running stress

Exercise around the lactate threshold induces a stress response, defined as "running stress." We have previously demonstrated that running stress is associated with activation of certain regions of the brain, e.g., the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus, that are hypothesized to play an integral role in regulating stress-related responses, including ACTH release during running. Thus we investigated the role of prolactin-releasing peptide (PrRP), found in the ventrolateral medulla and the nucleus of the solitary tract, which is known to project to the PVN during running-induced ACTH release. Accumulation of c-Fos in PrRP neurons correlated with running speeds, reaching maximal levels under running stress. Intracerebroventricular injection of neutralizing anti-PrRP antibodies led to increased plasma ACTH level and blood lactate accumulation during running stress, but not during restraint stress. Exogenous intracerebroventricular administration of low doses of PrRP had the opposite effects. Therefore, our results suggest that, during running stress, PrRP-containing neurons are activated in an exercise intensity-dependent manner, and likewise the produced endogenous PrRP attenuates ACTH release and blood lactate accumulation during running stress. Here we provide a novel perspective on understanding of PrRP in the endocrine-metabolic response associated with running stress.

Airjet and FG-7142-induced Fos expression differs in rats selectively bred for high and low anxiety-related behavior

We reported recently that two rat lines bred for either high (HAB) or low (LAB) anxiety-related behavior display differential Fos expression in restricted parts of the fear/anxiety circuitry when exposed to mild anxiety evoked in exploratory anxiety tests. Since different forms of anxiety are thought to activate different parts of the anxiety circuitry, we investigated now whether (1) an aversive stimulus which elicits escape behavior (airjet) and (2) the anxiogenic/panicogenic drug FG-7142 would reveal further differences in Fos expression as a marker of neuronal activation between HAB and LAB rats. Both airjet exposure and FG-7142 induced Fos expression in both lines in various anxiety-related brain areas. HAB rats, which displayed exaggerated escape responses during airjet exposure, exhibited increased Fos expression in brain areas including the hypothalamus, periaqueductal gray and locus coeruleus, as well as blunted Fos activation in the cingulate cortex in response to airjet and/or FG-7142. The results corroborate previous findings showing that trait anxiety affects neuronal excitability in hypothalamic and medial prefrontal areas. Furthermore, by using airjet as well as FG-7142, we now reveal that enhanced trait anxiety is also associated with neuronal hyperexcitability in the locus coeruleus and the periaqueductal gray, suggesting that investigation of an array of different anxiogenic stimuli is important for the detection of altered neuronal processing in trait anxiety.

Cardiovascular effects and c-Fos expression in the rat hindbrain in response to innocuous stomach distension

The present work was planned to study the effects of non-noxious gastric distension on hemodynamic variables and on cardiovascular hindbrain areas detected by means of c-Fos immunoreactivity, to determine the afferent and central mechanisms involved. In anesthetized rats, innocuous stomach distension increased arterial blood pressure and heart rate and induced c-Fos immunoreactivity within nucleus tractus solitarii, nucleus ambiguus, ventrolateral medulla and lateral reticular nucleus. Bilateral vagotomy abolished the pressor response and c-Fos immunoreactivity in nucleus ambiguus and ventrolateral medulla. Also, c-Fos immunoreactivity was significantly decreased in nucleus tractus solitarii and lateral reticular nucleus. After bilateral splanchnicotomy the pressor and tachycardic responses caused by gastric distension were reduced. c-Fos immunoreactivity in nucleus tractus solitarii, lateral reticular nucleus and nucleus ambiguus was reduced in comparison to the intact rats. In ventrolateral medulla a preferential localization of c-Fos immunoreactivity was found within its caudal portion. It was shown that such gastric distension, known to activate low threshold mechanoreceptors, induced cardiovascular effects via both vagal and splanchnic afferents and involving their central convergence and interaction in modulating the baroreceptor buffer system.

Inhibition of nociceptive responses of spinal cord neurones during hypertension involves the spinal GABAergic system and a pain modulatory center located at the caudal ventrolateral medulla

The mechanisms of hypertension-induced hypoalgesia were studied in a model of hypertension induced by adenosine receptors blockade with the non-selective antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) during 7 days. Based on the positive correlation between pain thresholds and noxious-evoked expression of the c-fos protooncogene in spinal cord neurones, we used this marker of nociceptive activation of spinal neurones to evaluate the involvement of the spinal GABAergic system and the caudal ventrolateral medulla (VLM), an important inhibitory component of the supraspinal endogenous pain modulatory system. In DPSPX-treated animals, a 20% increase in blood pressure was achieved along with a decrease in Fos expression in the superficial (laminae I-II) and deep (laminae III-VII) dorsal horn. In these animals, lower percentages of neurones labeled for GABAB receptors that expressed Fos were obtained in the superficial dorsal horn. Lesioning the VLMlat with quinolinic acid prevented the decrease in Fos expression at the spinal cord of DPSPX-hypertensive rats whereas in normotensive animals, no changes in Fos expression were detected. The present results support previous findings that hypertension is associated with a decrease of nociceptive activation of spinal cord neurones, through descending inhibition exerted by the VLMlat. This study further shows that during hypertension a decrease in the expression of GABAB receptors in nociceptive spinal neurones occurs, probably due to changes in the local GABAergic inhibitory system.

Hemorrhage activates proopiomelanocortin neurons in the rat hypothalamus

Severe blood loss lowers arterial pressure through a central mechanism that is thought to include opioid neurons. In this study, we investigated whether hemorrhage activates proopiomelanocortin (POMC) neurons by measuring Fos immunoreactivity and POMC mRNA levels in the medial basal hypothalamus. Hemorrhage (2.2 ml/100 g body weight over 20 min) increased the number of Fos immunoreactive neurons throughout the rostral-caudal extent of the arcuate nucleus, the retrochiasmatic area and the peri-arcuate region lateral to the arcuate nucleus where POMC neurons are located. Double label immunohistochemistry revealed that hemorrhage increased Fos expression by beta-endorphin immunoreactive neurons significantly. The proportion of beta-endorphin immunoreactive neurons that expressed Fos immunoreactivity increased approximately four-fold, from 11.7+/-1.4% in sham-operated control animals to 42.0+/-5.2% in hemorrhaged animals. Hemorrhage also increased POMC mRNA levels in the medial basal hypothalamus significantly, consistent with the hypothesis that blood loss activates POMC neurons. To test whether activation of arcuate neurons contributes to the fall in arterial pressure evoked by hemorrhage, we inhibited neuronal activity in the caudal arcuate nucleus by microinjecting the local anesthetic lidocaine (2%; 0.1 or 0.3 microl) bilaterally 2 min before hemorrhage was initiated. Lidocaine injection inhibited hemorrhagic hypotension and bradycardia significantly although it did not influence arterial pressure or heart rate in non-hemorrhaged rats. These results demonstrate that hemorrhage activates POMC neurons and provide evidence that activation of neurons in the arcuate nucleus plays an important role in the hemodynamic response to hemorrhage.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Activation of A1 and A2 noradrenergic neurons in response to running in the rat

Since running accompanied with blood lactate accumulation stimulates the release of adrenocorticotropic hormone (ACTH), running above the lactate threshold (LT) acts as stress (running stress). To examine whether A1/A2 noradrenergic neurons that project to the hypothalamus activate under running stress, c-Fos immunohistochemistry was used to compare the effects of running with or without stress response on A1/A2 noradrenergic neurons. Blood lactate and plasma ACTH concentrations significantly increased in the running stress group, but not in the running without stress response and control groups, confirming different physiological impacts between different intensity of running with or without stress. Running stress markedly increased c-Fos accumulation in the A1/A2 noradrenergic neurons. Running without stress response also induced a significant increase in c-Fos expression in the A1/A2 noradrenergic neurons, and the percentage of the increase was smaller than that of running stress. The extent of c-Fos expression in the A1/A2 noradrenergic neurons correlates with exercise intensity, signifying that this neuronal activation is running speed-dependent. We thus suggest that A1/A2 noradrenergic neurons are activated in response to not only running stress, but also to other physiological running, enhanced by non-stressful running. These findings will be helpful in studies of specific neurocircuits and in identifying their functions in response to running at different intensities.

Stimulation of cardiac sympathetic afferents activates glutamatergic neurons in the parabrachial nucleus: relation to neurons containing nNOS

Our previous studies have demonstrated that stimulation of cardiac sympathetic afferents activates neurons in the parabrachial nucleus (PBN), a region known to play a role in central integration of cardiovascular autonomic reflexes. However, phenotypes of these activated neurons have not been well identified. Glutamate, an important excitatory neurotransmitter in the brain, is involved in PBN-mediated cardiovascular responses. Recent identification of vesicular glutamate transporter 3 (VGLUT3) has provided a novel and unique marker to locate distinctive perikarya of neurons that use glutamate as a neurotransmitter. The action of glutamate in the brain is influenced by nitric oxide. Thus, using triple immunofluorescent labeling, the present study examined expression of c-Fos, an immediate early gene, in the neurons containing VGLUT3 and neuronal nitric oxide synthase (nNOS) in the PBN following stimulation of cardiac sympathetic afferents. In anesthetized cats with bilateral barodenervation and cervical vagotomy, topical application of bradykinin (BK, 1-10 microg/ml, 50 microl, n = 6) on the left ventricle was performed six times, every 20 min. Repeated administration of BK elicited consistent increases in blood pressure over a 100 min period while no changes were noted in the animals treated with the vehicle for BK (0.9% saline, n=5). Compared to control cats, c-Fos expression was increased significantly in the cell bodies containing VGLUT3 as well as both VGLUT3 and nNOS in the external lateral PBN (elPBN) in BK-treated animals (all P < 0.01). In addition, using similar triple-staining method, we noted that fibers of activated neurons containing nNOS in the elPBN co-localized with vesicular glutamate transporter 2 following BK stimulation. These data suggest that glutamatergic neurons represent a cell type in the PBN that is activated by stimulation of cardiac sympathetic afferents. Nitric oxide has the potential to influence the action of glutamatergic neurons in regulation of excitatory cardiovascular responses induced by activation of cardiac sympathetic afferents.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

α-Lipoic acid treatment prevents the diabetes-induced attenuation of the afferent limb of the baroreceptor reflex in rats

Autonomic neuropathies, common complications of prolonged diabetes, may result from diabetes-induced increased oxidative stress. Recently, we found that the afferent component of the baroreceptor reflex is attenuated in streptozotocin-induced diabetic rats. This study sought to determine the influence of the anti-oxidant, alpha-lipoic acid on the diabetes-induced deficits of the afferent limb of the baroreceptor reflex and on plasma malondialdehyde (a measure of lipid peroxidation). The number of c-Fos-ir neurons in the nucleus tractus solitarius in response to phenylephrine-induced baroreceptor activation was used as an index of the integrity of the afferent limb of the baroreceptor reflex. Groups of streptozotocin-induced diabetic and non-diabetic control rats, maintained from 8 to 16 weeks, were treated with alpha-lipoic acid (100 mg kg(-1) IP, 5x/week), or vehicle for the last 4 weeks prior to the experimental procedure. Vehicle-treated diabetic rats had elevated plasma malondialdehyde levels when compared to non-diabetic rats. alpha-Lipoic acid-treated diabetic rats had plasma malondialdehyde levels similar to those seen in non-diabetic rats and less than those of vehicle-treated diabetic rats at both the 8- and 16-week time points.alpha-Lipoic acid treatment did not affect the baseline (absence of baroreceptor activation) presence of c-Fos-ir in the nucleus tractus solitarius. In response to phenylephrine and regardless of treatment, the diabetic and control rats displayed increases in blood pressure and reflex bradycardia. As previously reported, phenylephrine-induced baroreceptor activation resulted in significantly fewer c-Fos-ir neurons in the nucleus tractus solitarius (commissural and caudal subpostremal regions) of diabetic rats when compared to non-diabetic rats at both 8- and 16-week time points. Four weeks of alpha-lipoic acid treatment reversed the diabetes-induced decrement in the numbers of c-Fos-ir neurons in the nucleus tractus solitarius in response to baroreceptor activation. alpha-Lipoic acid-treated diabetic rats showed the same phenylephrine-induced c-Fos response in the nucleus tractus solitarius as those of alpha-lipoic-acid- and vehicle-treated control rats at both 8- and 16-week time points. These data suggest that diabetes-induced oxidative stress plays a role in diabetes-induced baroreceptor dysfunction and that the alpha-lipoic acid may have a beneficial effect in treatment of diabetic autonomic neuropathy.

Activation of nitric oxide-producing neurons in the brain stem during cardiac sympathoexcitatory reflexes in the cat

Our previous studies have shown that selective inhibition of nitric oxide in the brain reduces pressor responses to activation of cardiac sympathetic afferents, thus suggesting that nitric oxide is involved in central regulation of cardiac-cardiovascular sympathoexcitatory reflexes. Central neural regions in which nitric oxide-producing neurons are activated during these reflexes have not been well characterized. In the present study, we located nitric oxide-producing neurons in the brain stem activated by the input from cardiac sympathetic afferents by detecting colocalization of c-Fos immunoreactivity with nitric oxide synthesizing neurons. Expression of c-Fos has been used as a marker of activated neurons. Nitric oxide-producing neurons were identified by histochemical labeling of nicotine adenine dinucleotide phosphate-diaphorase (NADPH-d). In anesthetized cats with bilateral barodenervation and cervical vagotomy, bradykinin (1-10 microg in 0.1 ml; n=6) was applied to the anterior surface of the left ventricle six times every 20 minutes. Repetitive application of bradykinin consistently increased blood pressure, while the vehicle for bradykinin (0.9% saline, n=5) produced no responses. A substantial fraction (6-27%) of NADPH-d positive neurons displayed Fos immunoreactivity in the nucleus of the solitary tract, caudal and rostral ventral lateral medulla, lateral tegmental field, locus coeruleus and parabrachial nucleus in the bradykinin-treated cats. However, either no or rare (1-4%) double-labeled cells were found in these regions in control animals. Thus, nitric oxide-producing neurons are activated in several regions in the brain stem during stimulation of cardiac sympathetic afferents by bradykinin. Our data suggest that nitric oxide functions as a neurotransmitter/modulator in these areas to regulate the cardiac sympathoexcitatory reflexes.

Increased c-Fos expression in select lateral parabrachial subnuclei following chemical versus electrical stimulation of the dorsal periaqueductal gray in rats

The parabrachial nucleus (PBN) is located in the rostral dorsolateral pons and has been identified as a critical relay for cardiovascular responses (sympathoexcitation and baroreflex attenuation) evoked by the dorsal periaqueductal gray (PAG). We examined the pattern of c-Fos protein immunoreactivity throughout the rostral-caudal extent of the PBN in four groups of anesthetized male Sprague-Dawley rats to identify the specific PBN regions activated by dorsal PAG stimulation. Both electrical stimulation and chemical (0.3 mM bicuculline methobromide) activation of the dorsal PAG elicited a selective increase in Fos-like immunoreactivity (FLI) in the superior lateral and central lateral subnuclei of the rostral lateral PBN (LPBN) relative to surgery and blood pressure control groups. In the middle LPBN chemical stimulation of the dorsal PAG selectively increased FLI in the central lateral subnucleus while electrical stimulation increased FLI in the Kolliker-Fuse area only. Finally, in the caudal LPBN only electrical stimulation of the dorsal PAG induced significant changes in FLI above control. Significant changes in FLI in the medial PBN were not observed under any experimental conditions. These results confirm neuroanatomical data demonstrating that neurons in superior lateral and central lateral subnuclei of the rostral and middle LPBN are the primary targets of the dorsal PAG. Our results also demonstrate that this descending projection to the central lateral and superior lateral subnuclei of the LPBN is in part excitatory. Finally, our results raise the possibility that neurons in the central lateral subnucleus of the middle and rostral LPBN are integrally involved in descending modulation of sympathetic drive associated with dorsal PAG activation.

Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

c-Fos expression in the midbrain periaqueductal gray after chemoreceptor and baroreceptor activation

The pattern of Fos-like immunoreactivity (FLI) in the periaqueductal gray (PAG) associated with activation of arterial chemoreceptors versus baroreceptor afferents was examined in urethane-anesthetized rats. Chemoreflex responses elicited by repeat intravenous injections of potassium cyanide (KCN; 90 microg/kg) significantly increased FLI in all columns of the PAG relative to saline-injected animals. Pressor responses elicited by intravenous phenylephrine (PE) produced a similar pattern of increased FLI throughout the PAG except in the dorsomedial and lateral columns of the caudal PAG, where FLI was minimal. Chemoreflex responses were unaltered by blockade of excitatory amino acid receptors in the dorsomedial PAG, and < 10% of the neurons of the caudal PAG that expressed FLI after KCN stimulation were retrogradely labeled from the A5 region of the caudal ventrolateral pons. These results indicate that integration of chemoreceptor inputs occurs primarily in the dorsal and lateral columns of the caudal PAG, but these neurons have little direct descending influence over lower brain stem regions integral to the central arterial chemoreflex arc.

Central pathways in the pons and midbrain involved in cardiac sympathoexcitatory reflexes in cats

Activation of cardiac sympathetic afferents elicits pain and excitatory cardiovascular reflexes including acute hypertension and tachyarrhythmias. Our previous studies have shown that specific regions in the medulla, such as the nucleus of solitary tract and ventrolateral medulla, are involved in central regulation of cardiac sympathoexcitatory reflexes. However, the contributions of supramedullary nuclei to these reflexes have not been characterized. In the present study, we located activated neurons in the pons and midbrain induced by inputs from cardiac sympathetic afferents by detecting their c-Fos immunoreactivity. In anesthetized cats with bilateral carotid denervation and cervical vagotomy, epicardial application of bradykinin (1-10 microg, in 0.1 ml; n=7) was performed on the anterior surface of the left ventricle six times, every 20 min. Repetitive application of bradykinin caused consistent excitatory cardiovascular reflexes characterized by increases in blood pressure and heart rate. No responses were evoked by the vehicle for bradykinin (0.9% saline, n=7). Compared to control cats, c-Fos immunoreactive cells were significantly increased (P<0.05) in the rostral pons, caudal and intermediate midbrain in the bradykinin-treated cats. The specific areas activated include the parabrachial nucleus, Kölliker-Fuse nucleus, locus coeruleus, dorsal nucleus of raphe, and dorsal, lateral and ventrolateral periaqueductal gray. From these results we suggest that cardiovascular-related regions in the pons and midbrain form part of a long loop in central integration of cardiac sympathoexcitatory reflexes.

Serotonin(3) receptor stimulation in the nucleus tractus solitarii activates non-catecholaminergic neurons in the rat ventrolateral medulla

The present study was performed to determine whether or not the increased arterial pressure triggered by 5-HT(3) receptor stimulation in the nucleus tractus solitarii and underlain by a sympathoexcitation is associated with the activation of ventromedullary cells known to be involved in vascular regulation, i.e. the C1 and A1 catecholaminergic cells. For this purpose, double immunohistochemical labeling for tyrosine hydroxylase and c-fos protein was performed all along the ventrolateral medulla after microinjection of 1-(m-chlorophenyl)-biguanide, a selective and potent 5-HT(3) receptor agonist, into the nucleus tractus solitarii of alpha-chloralose/urethane-anaesthetized rats. This treatment produced a significant elevation of arterial pressure ( approximately +35 mm Hg). Concomitantly, a significant increase in the number of c-fos expressing neurons was observed in the rostral ventrolateral medulla (+63%), in particular in its most anterior part (+78%), and in the medullary region surrounding the caudal part of the facial nucleus (+91%). Retrograde labeling with gold-horseradish peroxidase complex showed that at least some of these activated c-fos expressing cells project to the spinal cord. However, the number of double-stained neurons, i.e. c-fos and tyrosine hydroxylase positive neurons, did not increase at any level of the ventrolateral medulla. In contrast, under the same alpha-chloralose/urethane anesthesia, systemic infusion of sodium nitroprusside appeared to produce a hypotension and a marked increase in the density of such double c-fos and tyrosine hydroxylase expressing cells in the rostral ventrolateral medulla and the caudal medullary region surrounding the caudal part of the facial nucleus. These data indicate that medullary catecholaminergic C1 and A1 neurons are not involved in the pressor effect elicited by 5-HT(3) receptor stimulation in the nucleus tractus solitarii. However, this 5-HT(3) receptor-mediated effect is clearly associated with the excitation of (non-catecholaminergic) neurons within the pressor region of the ventral medulla.

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

Neurochemistry of nerve fibers apposing sympathetic preganglionic neurons activated by sustained hypotension

Sympathetic preganglionic neurons (SPN) in rat spinal cord were activated by the reflex stimulation of bulbospinal sympathetic neuronal pathways after a nitroprusside-induced hypotension. Hypotension-sensitive SPN, identified by immunoreactivity (IR) to the product of the immediate early gene c-fos and to choline acetyltransferase, were localized in the intermediolateral cell column of thoracic and upper lumbar cord, particularly middle to lower thoracic cord. Putative neurotransmitters, or their markers, in varicose fiber networks around SPN were identified. Nearly all hypotension-sensitive (Fos-IR) SPN were apposed by varicose fibers immunoreactive for tyrosine hydroxylase, serotonin, substance P, or enkephalin. Neuropeptide Y (NPY)- or phenylethanolamine-N-methyl transferase (PNMT)-IR varicose fibers apposed Fos-IR SPN in the upper and middle thoracic spinal cord, but in lower thoracic segments some Fos-IR SPN lacked these appositions. In thoracic segment 12, 51% +/- 5% of Fos-IR SPN (n = 9 rats) lacked PNMT contacts and 25% +/- 3% of Fos-IR SPN (n = 8 rats) lacked NPY contacts. In contrast to other chemically defined afferents, galanin-IR varicose fibers apposed fewer than half of the Fos-IR SPN in the middle to lower thoracic cord. Neurotransmitters/neuromodulators that might influence the activity of SPN acting in the baroreflex-mediated control of blood pressure have been identified. Uniformity in the neurochemistry of some fibers making connections with Fos-IR SPN, regardless of their segmental origin, suggests that common sets of neurons provide convergent inputs to all hypotension-sensitive SPN. Other fibers show topographic differences in their contacts with Fos-IR SPN, suggesting that subgroups of hypotension-sensitive SPN are targeted by particular neuron groups.

Fos immunoreactivity in the diagonal band and the perinuclear zone of the supraoptic nucleus after hypertension and hypervolaemia in unanaesthetized rats

We used Fos immunocytochemistry to study the effects of hypertension and hypervolaemia on neurones in the diagonal band of Broca and the perinuclear zone of the supraoptic nucleus, two nuclei that are both involved in the baroreceptor regulation of vasopressin neurones in the supraoptic nucleus. In addition, we used sino-aortic denervation to examine the role of arterial baroreceptors in the response to these haemodynamic changes. Sham-operated and sino-aortic denervated rats were infused with phenylephrine sufficient to increase blood pressure for 2 h. Control rats were infused with the same volume of isontonic saline. Only Sham sino-aortic denervated rats showed reflex bradycardia in response to the increased blood pressure. Volume expansion was produced by infusing the rats with isotonic saline equal to 10% of their body weight for 10 min, which significantly increased central venous pressure. In the diagonal band of Broca and the perinuclear zone, the number of Fos-positive neurones was significantly increased after phenylephrine infusion. Sino-aortic denervation blocked the significant increase in both regions. After volume expansion, a significant increase in Fos staining was observed only in the perinuclear zone of the supraoptic nucleus. This increase was not blocked by sino-aortic denervation. Our results indicate that both the diagonal band of Broca and the perinuclear zone of the supraoptic nucleus are activated by stimulating arterial baroreceptors; however, the perinuclear zone of the supraoptic nucleus is stimulated during volume expansion. Furthermore, the activation of perinuclear zone of the supraoptic nucleus after volume expansion is not dependent on intact arterial baroreceptors.

Medullary pathways involved in cardiac sympathoexcitatory reflexes in the cat

Stimulation of cardiac sympathetic afferents evokes excitatory cardiovascular reflexes. However, the exact regions in the brain that integrate these reflexes have not been identified. Expression of c-Fos in the neurons provides a useful marker of the activated neurons. In the present study, we examined the response of c-Fos within the medulla of the cat to chemical stimulation of cardiac sympathetic afferents. After bilateral sinoaortic denervation and cervical vagotomy, we applied bradykinin (BK, 1-10 microg, n=7) six times to the anterior ventricular surface every 20 min. We observed consistent increases in blood pressure and heart rate while the vehicle for BK (0.9% saline, n=6) produced no responses. Ninety minutes after the end of the sixth treatment, transcardial perfusion was performed with 4% paraformaldehyde and the brainstem was harvested for immunohistochemical staining. Compared to the control animals, we noted Fos immunoreactive neurons in the nucleus of the solitary tract, lateral tegmental field, caudal and rostral ventrolateral medulla (VLM), and vestibular nucleus in the BK-treated cats (all P<0.05). Fos immunoreactivity was found in catecholaminergic neurons of the VLM. These findings indicate that the activated neurons in the medulla, especially in the VLM, are involved in integration of cardiac-cardiovascular sympathoexcitatory reflexes.

Projection sites of superficial and deep spinal dorsal horn cells in the nucleus tractus solitarii of the rat

By using anterograde transport of biotin dextran amine injected into the cervical spinal dorsal horn, we have shown that fibres from superficial and deep dorsal horn project to the nucleus tractus solitarii via two distinct pathways. Afferent fibres from the superficial lamina (I-III) were found to course in the dorsal funiculus and terminate bilaterally in the caudal zone of the nucleus tractus solitarii (NTS), mainly within the commissural subnucleus. In contrast, afferents from the deeper dorsal horn laminae (IV-V) were found to course in the dorsolateral fasciculus and terminate ipsilaterally, mostly in the lateral areas of the caudal nucleus tractus solitarii. Similar, but more extensive patterns of labelled fibres were produced by injections into the white matter of the dorsal funiculus and dorsolateral fasciculus, respectively. These observations suggest that the caudal NTS not only serves as a location of visceral afferent convergence and integration, but may also be a receptive area for monosynaptic projections from dorsal horn neurons receiving sensory afferent inputs. Such projections may represent pathways through which NTS neurons are influenced by nociceptive and non-nociceptive information from the dorsal horn and thereby can co-ordinate the appropriate autonomic response, including adjustments in cardiorespiratory reflex output.

Intracerebroventricular adrenomedullin stimulates the hypothalamic-pituitary-adrenal axis, the sympathetic nervous system and production of hypothalamic nitric oxide

We tested the hypothesis that central adrenomedullin stimulates activity of the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic output from the brain, and we assessed the effects of central adrenomedullin on the nitric oxide (NO) system in the brain. In conscious rats, intracerebroventricular (i.c.v.) injections of adrenomedullin (2 nmol/kg) increased arterial pressure and heart rate, with return to baseline values within 20 min and 65 min of injections, respectively. Adrenomedullin injections augmented expression of tyrosine hydroxylase mRNA in the locus coeruleus after 4 h. Plasma concentrations of adrenocorticotropic hormone, measured with radioimmunoassay, were also increased by adrenomedullin. i.c.v. Adrenomedullin stimulated Fos expression in neurones within autonomic centres including the paraventricular nucleus (PVN) of the hypothalamus, arcuate nucleus, locus coeruleus, nucleus of the tractus solitarius and area postrema. In the PVN, large proportions of corticotropin releasing factor- and NO-producing neurones were activated (Fos positive). NO production, measured with nitrate/nitrite assays, was elevated in the hypothalamus, but not brainstem, of adrenomedullin-treated rats compared to controls. We conclude that centrally administered adrenomedullin stimulates activity of the HPA axis, the sympathetic nervous system, and the hypothalamic NO system.

Olanzapine activates the rat locus coeruleus: in vivo electrophysiology and c-Fos immunoreactivity

BACKGROUND

Activation of central noradrenergic pathways by atypical antipsychotics has been hypothesized to play a role in their efficacy in treating the negative symptoms and cognitive impairment of schizophrenia. Because acute administration of the atypical antipsychotic olanzapine has been shown to increase extracellular levels of norepinephrine in the medial prefrontal cortex, we examined the effects of olanzapine on the noradrenergic cells of the locus coeruleus (LC).

METHODS

The effects of olanzapine (0.25-16 mg kg(-1), IV) on the firing rates and patterns of LC neurons were determined by extracellular, single-unit recordings in chloral hydrate-anaesthetized rats. The effects of olanzapine and clozapine on c-Fos expression in the LC, nucleus subcoeruleus part alpha (SubCA), and nucleus A5 (A5) were studied by immunohistochemistry.

RESULTS

Olanzapine increased LC cell firing rates, de-regularized firing, and induced burst firing. Induction of c-Fos expression in the LC by olanzapine and clozapine was confirmed and was also found in the SubCA, but not in A5.

CONCLUSIONS

Acute administration of olanzapine activates the noradrenergic neurons of the rat LC. This increased activity of LC neurons may play an important role in the efficacy of olanzapine and clozapine in treating both the negative symptoms and cognitive impairment observed in schizophrenic patients.

Attenuation of the afferent limb of the baroreceptor reflex in streptozotocin-induced diabetic rats

Diabetic autonomic neuropathy is a common complication following prolonged diabetes. Alterations of cardiovascular reflexes contribute to the increased cardiovascular morbidity and mortality seen in diabetic patients. This study sought to better characterize these complications by investigating the afferent limb of the baroreceptor reflex in an experimental rat model of diabetes. Streptozotocin (STZ)-induced diabetic and euglycemic control rats were studied at 8- and 16-week time points after initiation of the experiment. Activation of the afferent limb of the baroreceptor reflex was assessed by measuring the numbers of c-Fos-immunoreactive (ir) neurons in the CNS site of termination of the baroreceptor afferent neurons, the nucleus of the solitary tract (NTS). Initial experiments established that baseline cardiovascular parameters and NTS expression of c-Fos-ir neurons were not different between diabetic and control rats at either time point. Phenylephrine (PE)-induced activation of baroreceptors resulted in a significant elevation in the numbers of c-Fos-ir neurons in the NTS of control rats. Although diabetic rats showed similar pressor responses to PE, the activation of c-Fos-ir neurons in the NTS of diabetic rats was significantly attenuated. At both 8 and 16 weeks, STZ-induced diabetic rats had significantly fewer c-Fos-ir neurons in the commissural NTS and in the caudal subpostrernal NTS when compared to the non-diabetic control animals receiving PE. These data suggest that STZ-induced diabetes, for a period of 8 and 16 weeks, results in reduced activity in the afferent baroreceptor input to the NTS, and are consistent with diabetes-induced damage to baroreceptor afferent nerves.

Effects of selective sinoaortic denervations on phenylephrine-induced activational responses in the nucleus of the solitary tract

Intravenous administration of phenylephrine provokes a pattern of cellular activation in the nucleus of the solitary tract that resembles the central distributions of primary baroreceptor afferents supplied by the carotid sinus and aortic depressor nerves. Transganglionic transport and denervation methods were used in an experimental setting to test the dependence of phenylephrine-induced Fos immunoreactivity on the integrity of buffer nerve afferents, and to identify the subregions of the nucleus of the solitary tract supplied by each. Cholera toxin B-horseradish peroxidase injections into either or both nerves revealed terminal labeling concentrated in, but not restricted to, the dorsal commissural part of the nucleus of the solitary tract at the level of the apex of calamus scriptorius, and extending into the dorsal subnucleus at the level of the area postrema. Preferential ramifications of carotid sinus and aortic depressor nerve afferents at the levels of the commissural part of the nucleus and the area postrema, respectively, were reflected in the extent to which labeled fibers comingled with neurons exhibiting phenylephrine-induced Fos in dual labeling experiments. Complete sinoaortic denervation reduced by 90% the number of neurons exhibiting drug-induced Fos expression. Selective carotid and aortic sinus denervations effected partial reductions manifest preferentially in the caudal and rostral foci of the distribution, respectively. Reduced activational responses at the level of the area postrema of aortic sinus-denervated rats were accompanied by a reduction in cellular nicotinamide adenine dinucleotide phosphate-diaphorase activity in this region. Animals killed 30 days after complete sinoaortic denervation displayed no evidence of recovery of phenylephrine-induced Fos, while the strength and distribution of the response in rats that received selective carotid sinus denervation were indistinguishable from those seen in controls. These findings (i) support the dependence of phenylephrine-induced Fos expression on the integrity of carotid sinus and aortic depressor nerve afferents, (ii) provide anatomical and functional evidence that the two buffer nerves distribute differentially within the nucleus of the solitary tract, and (iii) implicate central reorganization as a likely basis for functional recovery of baroreflex mechanisms following partial sinoaortic denervation.

Disordered central cardiovascular regulation in portal hypertensive and cirrhotic rats

Portal hypertension due to either prehepatic portal hypertension or cirrhosis is associated with cardiovascular derangement. We aimed to delineate regulatory mechanisms in the brain stem cardiovascular nuclei in rat models of prehepatic portal hypertension and cirrhosis. Neuronal activation in the nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM) were assessed by immunohistochemical staining for the immediate-early gene product Fos. In the same sections, catecholaminergic neurons were counted by tyrosine hydroxylase (TH) staining. Ninety minutes after hypotensive hemorrhage (or no volume challenge), the animals were killed for Fos and TH medullary staining. These protocols were repeated after capsaicin administration. The NTS of unchallenged sham-operated rats had scant Fos-positive cells (3.6 +/- 0.4 cells/section), whereas hemorrhage significantly increased Fos staining (91.8 +/- 14). In contrast, the unchallenged portal hypertensive and cirrhotic groups showed increased Fos staining (14.3 +/- 5.8 and 32.8 +/- 2.8, respectively), which hemorrhage did not alter significantly. The numbers of TH-positive cells were similar in the three unchallenged groups; double labeling revealed that approximately 50% of TH-positive cells were activated by hemorrhage in the sham and cirrhotic rats but not the portal hypertensive rats. Similar patterns of Fos and TH staining were observed in the VLM. Capsaicin treatment not only significantly reduced the Fos-positive neuron numbers in portal hypertensive and cirrhotic rats but also attenuated hemorrhage-induced Fos and double-positive cells in both NTS and VLM. These results suggest that disordered trafficking in capsaicin-sensitive nerves and central dysregulation contribute to blunted cardiovascular responsiveness in cirrhosis and prehepatic portal hypertension.

Visceral inputs to neurons in the anterior hypothalamus including those that project to the periaqueductal gray: a functional anatomical and electrophysiological study

The present study was designed to examine peripheral, in particular noxious visceral, inputs to neurons in the hypothalamus that project to the midbrain periaqueductal gray. The induction of Fos protein was used to localize hypothalamic neurons that were activated by noxious visceral stimulation. This was combined with retrograde transport of fluorescent latex microspheres from identified "pressor" and "depressor" sites in the dorsolateral/lateral or ventrolateral columns of the periaqueductal gray. A second series of electrophysiological experiments examined the receptive field characteristics, including the incidence of viscerosomatic convergence, of neurons in the ventral part of the anterior hypothalamus. Noxious visceral stimulation (intraperitoneal acetic acid) induced Fos-like immunoreactivity in significantly more neurons in the hypothalamus than control stimuli (intraperitoneal saline and intravenous phenylephrine). Particularly high numbers of Fos-positive neurons were found in the paraventricular nucleus, the supraoptic nucleus and ventral regions of the anterior hypothalamus. When combined with retrograde tracing from "depressor" sites in the ventrolateral periaqueductal gray, the highest numbers of double-labelled neurons were localized in the paraventricular nucleus and the lateral area of the anterior hypothalamus. However, the regions that contained the greatest proportions of Fos-positive neurons that projected to "depressor" sites in the ventrolateral periaqueductal gray were the lateral area of the anterior hypothalamus and its rostral extension, the lateral preoptic area. Fewer double-labelled neurons were localized in the hypothalamus after retrograde transport from sites in the dorsolateral/lateral periaqueductal gray compared to the results obtained from injections of tracer in the ventrolateral periaqueductal gray. Furthermore, the numbers of Fos-positive hypothalamic neurons that projected to the dorsolateral/lateral periaqueductal gray were very similar in experimental and control animals. The electrophysiological study confirmed that a large proportion of neurons in and around the lateral area of the anterior hypothalamus can be driven by noxious visceral stimulation and demonstrated a high incidence of viscerosomatic convergence in these cells (66% of cells driven from somatic structures were also driven by electrical stimulation of the splanchnic nerve). Somatic receptive fields of these neurons were generally large, often including all four limbs and the face. The results of the functional anatomical and electrophysiological studies have identified neurons in an area of the ventral anterior hypothalamus that are a focus of nociceptive visceral input and which project to the midbrain periaqueductal gray, in particular to its ventrolateral column. These results are discussed in relation to the roles of the anterior hypothalamus and the different longitudinal columns of the periaqueductal gray in co-ordinating autonomic and sensory functions in response to visceral pain.

Cortical control of somato-cardiovascular integration: neuroanatomical studies

This paper will discuss experiments dedicated to the exploration of pathways linking the sensorimotor cortex (SMC) and the main bulbar nuclei involved in cardiovascular control: the nucleus tractus solitarius (NTS), the dorsal nucleus of the vagus (DMV) and the rostral ventrolateral medulla (RVLM). Results obtained through neurofunctional and neuroanatomical methods are presented in order to bring new answers to relevant points concerning somato-cardiovascular integration: firstly to show the ability of the SMC to influence neurons in bulbar cardiovascular nuclei, and secondly to identify pathways that transmit such influences. The neurofunctional approach, based on the identification of Fos-like immunoreactive neurons, indicated that the SMC has functional connections with cardiovascular bulbar nuclei. The neuroanatomical approach, which employed retrograde and anterograde axonal tracing methods, provided evidence of direct projections from the SMC to NTS/DMV and RVLM. Furthermore, experiments showed clearly that corticospinal neurons sent collaterals to bulbar cardiovascular nuclei, especially to the RVLM. Direct cortical projections to the NTS/DMV and the RVLM provide the anatomical basis for cortical influences on the baroreceptor reflex and sympathetic vasomotor mechanisms for blood pressure control, and support the hypothesis of cortical commands coupling somatic and cardiovascular outputs for action.

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.

Expression of c-fos-like immunoreactivity in the feline brainstem in response to isometric muscle contraction and baroreceptor reflex changes in arterial pressure

This study compared whether activation of muscle ergoreceptor afferents caused by isometric muscle contraction, activation of baroreceptor afferents induced by i.v. infusion of phenylephrine, or baroreceptor afferent inactivation, caused by carotid artery occlusion, elicit similar patterns of c-Fos induction in brainstem areas. Adult cats were anesthetized with alpha-chloralose, and in each case, the experimental intervention caused an increase in the arterial blood pressure. There were two sets of control experiments: in both, animals underwent the same surgical procedures but then either remained at rest for the entire study, or the tibial nerve was stimulated, as in the contraction group, following muscle paralysis with tubocurarine. Following the procedures, animals rested for 90 min to allow neuronal expression of c-Fos. Control cats showed very little c-Fos immunoreactivity (c-Fos-ir) in the brainstem. Muscle contraction induced c-Fos-ir expression mainly in the nucleus tractus solitarius, lateral reticular nucleus, lateral tegmental field, vestibular nucleus, subretrofacial nucleus, spinal trigeminal tract and in a lateral region of the periaqueductal grey (P 0.5-1.0). The majority of the c-Fos-ir was found in brainstem areas contralateral to the contracted muscle. In addition, muscle contraction induced c-Fos-ir in the dorsal horns of spinal segments L6-S1 on the ipsilateral side of the spinal cord. Phenylephrine infusion caused c-Fos-ir expression in the nucleus tractus solitarius, spinal trigeminal tract, solitary tract, and dorsal motor nucleus of the vagus. No c-Fos-ir was apparent in the periaqueductal grey. Carotid occlusions induced c-Fos-ir expression in the area postrema, nucleus tractus solitarius, solitary tract, and spinal trigeminal tract. Expression was bilateral. Areas that exhibited c-Fos-ir correspond to sites previously reported to release various neuropeptides in response to muscle contraction or carotid occlusions. These results indicate that the exercise pressor reflex and baroreflex activate similar, but not completely identical, sites in the brainstem.

Effect of 5-HT(1A) receptor ligands on Fos-like immunoreactivity in rat brain: evidence for activation of noradrenergic transmission

This study investigated the effects of 8-OH-DPAT and various other 5-HT(1A) receptor agonists on brain noradrenergic transmission using Fos-like immunoreactivity (Fos-LI) as a marker of neural activation. Administration of 8-OH-DPAT (0.1 and 1 mg/kg) induced a marked and dose-related increase in the number of cells positive for Fos-LI in the locus coeruleus (LC), the main source of noradrenergic projections to the forebrain. This effect was also induced by the non-selective, partial 5-HT(1A) receptor agonist buspirone (10 mg/kg). The effect of both 8-OH-DPAT (0.1 mg/kg) and buspirone (10 mg/kg) on Fos-LI in the LC was blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). The active S(-)-enantiomer of the partial 5-HT(1A) receptor agonist (+/-)-MDL 75005EF (1 mg/kg) also induced the expression of Fos-LI in the LC, whereas the inactive R(+)-enantiomer of (+/-)-MDL 73005EF at the same dose did not. In addition to the LC, 8-OH-DPAT (0.1 mg/kg) also induced a marked increase in Fos-LI in various forebrain areas including the medial prefrontal cortex (infralimbic and cingulate cortical areas). More detailed analysis of the Fos response to 8-OH-DPAT in the medial prefrontal cortex revealed that the effect was attenuated by pretreatment with a combination of the beta(1)- and beta(2)-adrenoceptor antagonists ICI 118551 (4 mg/kg) and metoprolol (4 mg/kg), but not the alpha(1)-adrenoceptor antagonist prazosin (5 mg/kg). Taken together, the present findings provide immunocytochemical evidence that 5-HT(1A) receptor agonists activate noradrenergic neurones in the LC and that this leads to increased noradrenergic transmission at postsynaptic sites in the forebrain (specifically medial prefrontal cortex).